Bonsly is the weakest user of ember, doing about 54 - 64.8% in scenario one and 0.3% in scenario two. Bonsly cannot learn ember inherently, but it can learn mimic, allowing it to mimic ember and use it then. Lets get into the details.
The first scenario is somewhat unclear, but we'll assume kartana has a jolly nature, the opposing Pokemon a timid one, and run standard sweeping sets of 252 Spa/Atk, 252 Spe, and 4 SpD with no item on either. Bagon and fletchling tie for the weakest inherent ember user, and you can argue fletchling is weaker because bagon gets the ability sheer force. However, smeargle and bonsly can also learn ember via other means via sketch and mimic respectivly. We'll use bonsly for our purposes, but have the fletchling calculations on standby. In the end we end up doing 80.3 - 95.7% with fletchling and 54 - 64.8% on bonsly. In the event both Pokemon were meant to be jolly, we get 72.5 - 86.4% with fletchling and 49.4 - 58.6% on Bonsly. That's scenario one, but what about scenario two?
The first thing I did was set the EVs to make kartana extremely specially defensive with an EV spread of 252 HP, 4 Atk, 252 SpD, and make fletchling and bonsly 0 Spa and having a timid nature, the damage being 13.6 - 16.1% on bonsly and 27.3 - 32.2% with fletchling. The natures are the next step, giving kartana a calm nature and fletchling and bonsly a jolly one. We end up doing 27.3 - 32.2% with fletchling and 11.1 - 13.6% with bonsly. Then we simply remove the IVs from the ember users next, giving us 16.1 - 19.8% with fletchling and 6.2 - 7.4% on bonsly. I next added light screen and rain on the field, give kartana thick fat via skill swap and an assault vest, doing 0.3 - 0.6% with bonsly and 1.8 - 2.4% on fletchling. Now lets finally adjust the levels, making fletchling and bonsly level 1 and keeping kartana at level 100. This makes ember do 0.3 - 0.6% on both. This is because you can only do so little damage in one attack, and we are approaching that point. 0.3% is one damage and 0.6% is 2 damage. Now stat changes will be added, making them both -6 special attack, and kartana +6 Special defense. The damage amount is still the same on both, 0.3 - 0.6%. The final change we will make before concluding is giving kartana a water tera type, doing 1 damage.
Source: The showdown damage calculator and VilePlumer for correcting me that bonsly is the weakest user of ember.
Disclaimer: in the original post, it mentions that for scenario two, you can adjust stats, which I assume means stat changes like what cosmic power gives you, not actual base stats otherwise it would be more like blissey than kartana. I also put in regular rain instead of heavy rain, so ember would do some damage, and assumed we were in a single battle, not a double battle, so no friend guard. I will also mention water sport, but that isn't in the damage calculator settings, and 0.3% percent ends up being 1 damage, so it doesn't really matter too much. I also could have done guard split, but that was not used for the same reason as water sport.
